The present invention relates to a method and apparatus for forming an image by a digital technology. More particularly, the invention relates to an image forming apparatus, e.g., a printer or a digital copying machine, which uses the image forming method and apparatus to improve the gradation and graininess of a half-tone image, e.g., a xerographic image, by use of toner particles of dark color and light color.
In an image forming apparatus, such as a laser beam printer, a digital copying machine, or a facsimile machine, to the primary scan, a light beam, e.g., a laser beam, emitted from a semiconductor laser device, for example, is deflected in a predetermined direction by means of a rotating polygon, e.g., a polygon mirror, and to the secondary scan, a photosensitive member, i.e., a photoreceptor, is moved in the direction perpendicular to a predetermined direction on the image plane of the photoreceptor with respect to the light beam reflected by the polygon mirror or the light beam is deflected by an element, e.g., a galvano mirror. A scan speed is corrected by a f.theta. lens, for example, and the photoreceptor is two-dimensionally exposed to the light beam. The light beam is on/off controlled or interrupted in synchronism with the primary and secondary scanning motions, whereby an image is formed on the photoreceptor (the surface of the photoreceptor is imagewise exposed to the light beam). In other words, the uniformly charged surface of the photoreceptor is exposed to the light beam whose light amount is varied in accordance with optical densities of an image to be recorded or printed on the photoreceptor. As a result, the charge is removed from the photoreceptor surface in accordance with the optical densities of the image, to thereby form an electrostatic latent image on the photoreceptor surface. Then, toner particles are attached to the latent image thus formed on the photoreceptor surface.
A recent market in this field demands the improvement of the print quality of a half-tone image like as the images of characters and lines as binary images of black and white. A half-tone image forming apparatus is known in which the on- and off-times (durations) of the light beam moved at a constant speed in the primary scan direction, are controlled (the apparatus is based on the light-emission-time-duration control, or pulse width modulation method). More exactly, to form a half-tone image, the size of each dot to be printed is controlled by varying an amount of light beam (light emitting time X light intensity). In this case, to vary the amount of the light beam, the pulse width (light emission time) of the light beam or an intensity of light beam is varied.
Two methods are known to form a full color image by a xerography process. A first xerography-basis image forming method follows: a toner image is formed on the surface of a photoreceptor through a process of charging, exposure and development; the toner image is transferred onto a printing paper electrostatically attracted on the intermediate image transfer member; the image forming process and the image transferring process are sequentially repeated a plural number of times to form a full color toner image on the printing paper; and finally the color toner image is thermally fuzed and fixed onto the paper. A second xerography-basis image forming method uses xerography engines (each including photoreceptor, charging means, exposing means, developing means and image transferring means) whose number is equal to the number of colors of toner particles. The second method is capable of forming a multi-color image without reducing the printing speed even if the number of toner colors is increased. Thus, in the second method, the printing speed is not reduced if the number of toner colors is increased, but the apparatus size is increased since the number of xerography engines is increased.
The human eyes are sensitive to a minute variation (gradation) of optical density and the texture of grains (graininess) particularly in a high light portion of low density, e.g., skin color. For this reason, to improve the print quality of the half-tone image, it is necessary to improve the gradation and the graininess in the high light portion.
There are proposed techniques to form the half-tone image of the improved print quality (the Unexamined Japanese Patent Application Publication No. Sho 52-147444 and 58-162970). The first technique is that two kinds of toner particles which are equal in polarity but different in color are mixed in a state that their tribocharge amounts are different, and an electrostatic latent image, after formed, is developed using those kinds of toner particles to form an image of a multiple of colors. The second technique is that different kinds of toner particles which are different in density and electric resistance are mixed, and an electrostatic latent image, after formed, is developed using those kinds of toner particles, to produce a toner image of a reduced .gamma.-characteristic, or a half-tone image excellent in gradation. In those proposals, it is necessary to use the number of kinds of toner particles, which depends on the number of colors and gradation levels. This results in complexity and increase of size.
To produce a half-tone image at a desired density, a technique to set a ratio of the amount of black toner to the amount of the whole toner in accordance with a document density and a developing potential (reference is made to the Japanese Patent No. 2613649 corresponding to the Unexamined Japanese Patent Application Publication No. Hei 2-287370). Two kinds of developers (a developer for gradation which contains small amount of carbon black for gradation and a developer for high contrast which contains large amount of carbon black) are contained in separate developing units. To develop, one of the developing units is selected according to an instruction by an operator, which depends on the kind of document to be reproduced (the Unexamined Japanese Patent Application Publication No. Hei 1-297675). In the technique, it is necessary to set the toner amount ratio in accordance with a density of the half-tone image, and to change the setting in accordance with the type of document to be reproduced. In other words, the apparatus operation is complicated.
Another technique is disclosed in the Unexamined Japanese Patent Application Publication No. Sho 62-89073 and Sho 62-90679. In the technique, two kinds of developers (one developer containing black toner and the other containing gray toner) which are equal in hue but different in color tone are contained in separate developing units. A uniform exposure unit is disposed between the two developing units. An electrostatic latent image, after formed, is developed by use of those developers. More exactly, the uniformly charged photoreceptor is exposed to a light beam containing image information to thereby form an electrostatic latent image. The latent image is developed by use of the developing unit containing black toner, and the toner image is subjected to a uniform exposure by the uniform exposure unit. Thereafter, the toner image uniformly exposed is developed again by use of the developing unit containing gray toner. The resultant toner image accrues to a reproduced picture excellent in gradation. Specifically, the formation of an electrostatic latent image is performed in a state that a latent image potential in the white portion is increased by slightly reducing a light amount of the light beam. A latent image, after formed, is developed by the first developing unit, and subjected to a uniform exposure by the uniform exposing process, and the thus uniformly exposed latent image is developed by the second developing unit. At this time, the amounts of black toner and gray toner attached to the latent image or of the toner image are different from each other. The toner amount difference causes a potential difference between the latent image, and in turn a gradation difference of the image in level. In the technique, the uniform exposure process is essential and hence this makes the apparatus construction complicated.